Multiple-input and multiple-output (MIMO) is a feature of Long Term Evolution (LTE), which is a fourth generation (4G) wireless communication standard for providing high-speed data for mobile phones and data terminals. MIMO is based on the use of multiple antennas at both the transmitter and receiver to improve communication performance, e.g., by providing multiple radio channels. Multiuser MIMO (MU-MIMO) communications is one example of MIMO, wherein a base station (eNB) may simultaneously communicate with several user equipments (UEs), provided that substantial spatial insulation between the UEs exists.
Introduction of this kind of communications affects performance of data rate adaptation mechanisms. For example, selection of an appropriate modulation and coding scheme (MCS) for downlink (DL) communication with a UE is currently based on channel quality information (CQI) report from that UE. Appropriate CQI measurements are performed in the assumption of single-user MIMO (SU-MIMO), which may cause mismatch between the reported CQI and the real CQI that the UE will experience during MU-MIMO communication. The mismatch appears because the additional link(s) to other UE(s) create interference onto the UE that reported the CQI, and this interference is not known to the UE during CQI measurements.
Accordingly, eNB is likely to assign optimistic data rate (modulation and coding scheme) for the UE, which leads to increased error level for PDSCH transmission to the UE. Current LTE specifications provide so called “outer loop” mechanism to handle this increased error level. This mechanism tracks the ACK/NACK messages from the UE and based on their statistics adjusts the data rate of PDSCH sent to the UE. Nevertheless, this mechanism is relatively slow since it involves gathering error statistics over several tens of data packets delivered to the UE.